Many types of fuel filters and separators are known in the prior art. A popular type of filter and/or separator construction is a type that has a filter head to which a replaceable "spin on" element is attached. The head is a permanent part of the fuel system of the vehicle and includes inlet and outlet connections to the fuel lines. The element may be readily removed from the filter head and a new one attached without opening the fuel line connections to the filter head.
Problems may arise when the element is replaced. The old element which is removed, is filled with fuel. The new element when it is installed, is not. Therefore, the new element introduces an air pocket into the system. This air pocket may prevent fuel flow if fuel is drawn through the head and element by vacuum produced by a downstream fuel pump. However, even if the fuel is pushed through the element and the filter head, a large slug of air in the system may cause damage to downstream components such as an injector pump or fuel injectors.
Skilled mechanics sometimes attempt to fill a new element with clean fuel before it is installed. This reduces the size of the air pocket but does not eliminate the problem.
To eliminate the air pocket in a replacement filter or separator element, others have installed priming pumps on a filter head. Such priming pumps are of two types. The first type has two flow paths for fuel through the filter head. The first flow path passes fuel from the inlet of the head directly to a fluid in-flow area of an attached element. The fuel normally flows along this first path when the engine of the vehicle is running.
When the element has been replaced and there is a need to remove an air pocket from the system, a manual valve on the filter head is actuated to select the second flow path. The second flow path directs fuel from the inlet to a pumping chamber. The pumping chamber may be a variable volume area of a manually actuated piston or diaphragm pump. Appropriate valving is used to draw fuel from the inlet into the variable volume area of the pump and then to deliver the fuel from the pump to the in-flow area of the filter.
The pumping of fuel into the in-flow area of the filter forces liquid fuel into the element. The liquid displaces the air which may be either exhaust through a vent in the filter head or forced from the outlet of the head and moved down the fuel line to an area beyond the fuel pump or other components that could be damaged by air in the line.
Once the air pocket has been displaced the manual valve on the head is returned to its original position. In this condition fuel again passes directly from the inlet of the head to the fuel in the in-flow area of the element.
A disadvantage of this construction is that it requires the use of a manual valve in the filter head. The operator must remember to actuate this valve during priming and return the valve to its original condition after the priming operation is complete. Failure to return the valve to its original position may cause a malfunction. The pump and valve construction also adds significantly to the cost of the filter head.
An alternative type of prior art filter head construction includes a priming pump directly in the fluid flow path between the inlet to the head and the in-flow area of the element. In this design fluid passes through a first check valve on the downstream side of the pumping chamber that allows fluid to flow only into the pumping chamber from the inlet. A second check valve is positioned in the fluid path on the opposite side of the pump to allow fluid to flow only out of the pumping chamber.
When it is necessary to prime the element, the pump is actuated and the valves operate to force fuel into the in-flow area of the element. After priming, when the engine is started, a vacuum is drawn on the element if the fuel pump is positioned downstream of the filter head. The vacuum is sufficient to open flow through the check valves. If a fuel pump is positioned upstream from the filter head, the fluid pressure on the inlet of the filter head is sufficient to open flow through the check valves.
A problem with this configuration is the cost associated with building a head with flow passages, a pump and check valves. A further problem is that the check valves which are always in the flow path, pose restrictions to fluid flow when the engine is in the normal running condition. Thus, there exists a need for a filter head for mounting a filter or separator element that includes an integral priming pump that is inexpensive to manufacture, easy to use and has low flow resistance.